1. Field of the Invention
This invention relates to coating methods and apparatus. More particularly, it concerns a rack for supporting work pieces, such as screens in which the surfaces to be coated are interrupted by a high percentage of open area, as well as to a coating method using the rack.
2. Description of the Related Art
In the coating of screens or screen-like objects, the efficiency of coating becomes increasingly important as the cost of the coating material increases. One example occurs in the electrostatic photocopying art, in which scorotrons or other corona charging devices are used to charge or precharge photoreceptors. A negative precharging is used to neutralize the positive charge remaining on the photoreceptor after transfer of the developed toner image to a copy sheet and during cleaning to prepare the photoreceptor for the next copying cycle.
Certain difficulties have been observed to result from such a process. For example, it is believed that various nitrogen oxide species are produced by the corona and that these nitrogen oxide species are adsorbed by solid surfaces. In particular, it is believed that these nitrogen oxide species are adsorbed by the conductive elements as well as the housing of the corona generating device.
When a photocopying machine is turned off for extended periods of idleness, the adsorbed nitrogen oxide species are gradually desorbed. That is, the adsorption is a physically reversible process The adsorbed and desorbed species are both nitrogenous but are not necessarily the same, i.e., for example, there may be some conversion of NO.sub.x to HNO.sub.3. When operation of the machine is resumed, a copy quality defect is observed in the copies produced in that a line image deletion or lower density image is formed across the width of the photoreceptor at that portion of the surface of the photoreceptor which was at rest opposite the corona generating device during the period of idleness.
In a particular solution to this problem, as discussed in U.S. Pat. No. 4,920,266 issued to Reale, the screen of the corona generating device is coated with a substantially continuous thin conductive dry film of aluminum hydroxide containing particulate graphite and powdered nickel to inhibit adsorption of the nitrogen oxide species. These coating materials tend to be very expensive.
Scorotron grids must be given a substantially continuous coating of such materials to assure that they will function properly and must have opposite ends, which are masked during the coating process, that remain uncoated to form a good electrical contact with the photocopying device.
One conventional method of coating scorotron grids comprises a rack which includes a rectangular frame suspended by a pair of hooks and having four rectangular openings. Metal plates are pivotally connected to the frame at opposite sides of the openings by hinges. The metal plates are clamped against the front of the frame by a plurality of toggle clamps. The toggle clamps are mounted to the frame in a manner to securely press the plates against the frame.
In the conventional method of spray coating a scorotron grid using the rack described above, sheets of scorotron grids are placed against the rack with their ends disposed between the metal plates and the frame The toggle clamps are closed to pinch each sheet of scorotron grids between the plates and the frame such that each sheet of scorotron grids is thus secured to the rack. The plates also serve the purpose of masking the ends of the scorotron grids so that the ends of the grids are not spray coated.
The rack, with the sheets of scorotron grids attached, is then hung by the hooks from a rod, pipe, or similar hanging means, with a front side of the rack facing a spray gun, for example. The scorotron grids are then sprayed moving the spray gun along the length of a first scorotron grid on the sheet, moving the gun to the next scorotron grid, and then moving the gun along the length of the new grid.
When all of the scorotron grids have been coated, the rack is removed from the hanging means, rotated 180.degree. around a vertical axis, and hung by the hooks from the hanging means such that a back side of the rack opposite the front side now faces the spray ray gun. The back side of the scorotron grids, now facing the spray gun, is sprayed in the same manner as described above with respect to the spraying of the front side of the grids.
The rack is then removed from the hanging means, and the toggle clamps are opened, thereby releasing the sheets of scorotron grids from the rack.
The conventional device and method for spray coating mesh or screen parts such as scorotron grids have suffered from a series of problems. Since a large portion of the parts are open area, 70% or more in many cases, a large amount of spray coating material is wasted when it passes through the open areas of parts, thereby making the process of spray coating very inefficient and wasteful. Since the spray coating material used on the scorotron is expensive, the process is economically inefficient. Also, the coatings produced by the conventional methods often are not substantially continuous near the open areas, or holes, in the parts. When the coating is not substantially continuous, the parts must be resprayed, thereby creating more waste and inefficiency.
Additional problems with quality and efficiency arise because, over time, the spray coating adheres to the toggle clamps and hinges, making them stick and become difficult to open and close. This "sticking" slows the process, thereby making it more inefficient, and causes difficulties in achieving a proper masking on the ends of the scorotron grids since the plates do not then close tight against the rack.
An alternative conventional method of spray coating mesh or screen parts is electrostatic spraying. However, in applications such as spray coating scorotron grids which have a large percentage of open area but have small holes, 3 mm for example, the electrostatic spraying fails to coat the parts adequately because the electric field lines fail to penetrate the small holes in the grid. Also, special safety equipment concerns, when spraying conductive waterbase coatings, add to the process difficulties.